Nonaqueous electrolyte secondary batteries typified by lithium ion secondary batteries have high energy density and high capacity and therefore are used as driving power supplies for mobile data terminals such as mobile phones and notebook personal computers, electric vehicles, and the like.
Among the mobile data terminals, smartphones and tablet computers, which have been increasingly demanded in recent years, mostly have a structure in which a battery cannot be readily removed from a device body. Therefore, a nonaqueous electrolyte secondary battery for use in them is required to have higher energy density and longer life as compared to nonaqueous electrolyte secondary batteries used in conventional battery packs which are readily removable and replaceable.
Herein, in order to increase the energy density of a nonaqueous electrolyte secondary battery, for example, the packing density of an active material is increased by raising the rolling pressure during the manufacture of electrodes.
However, in the case of using graphite particles as a negative electrode active material, raising the rolling pressure allows basal planes parallel to the transverse direction of graphite crystals to be oriented in parallel to the transverse direction of a negative electrode and edge planes parallel to the stacking direction of the graphite crystals to be unlikely to be oriented to a surface of a negative electrode mixture layer (the degree of orientation is increased). Furthermore, the graphite particles are deformed or are cleaved, whereby the degree of orientation is further increased.
For example, in a lithium ion secondary battery, lithium ions are intercalated between layers from edge planes of graphite crystals. Therefore, when the degree of orientation is high, the lithium ions are unlikely to be stored in graphite particles; hence, input characteristics are reduced. The deformation of the graphite particles reduces the number of pores in a surface of a negative electrode mixture layer to further reduce the input characteristics.
Furthermore, the graphite particles are expanded and contracted by charge and discharge, respectively. This expands and contracts the negative electrode mixture layer. Therefore, repeating a charge/discharge cycle causes a problem that the contact resistance between negative electrode active material particles is increased and cycle life is reduced.
Therefore, in order to enhance battery characteristics of conventional nonaqueous electrolyte secondary batteries using a carbon material as a negative electrode active material, techniques disclosed in Patent Literatures 1 to 9 have been proposed.